This invention relates to methods and apparatus for liquid-liquid extraction.
Liquid-liquid extraction has been used for many years to transfer material dissolved in one liquid to another liquid. The two liquids are of different densities, and are substantially mutually insoluble so they can be intimately mixed to effect efficient transfer of the desired material from one liquid to the other. After the transfer is made, the two liquids are separated by gravity or centrifugal action.
In one application of liquid-liquid extraction, a material which is soluble in organic solvents is removed (extracted) from an aqueous solution by intimately contacting the aqueous phase with a suitable water-insoluble organic liquid as the extractant, followed by phase separation. Similarly, acidic or basic components of an organic solution can be removed by contacting it with an alkaline or acidic aqueous solution, as appropriate.
In another use of liquid-liquid extraction, an aqueous solution containing an ionic material (such as dissolved copper) is contacted with a liquid xe2x80x9cion exchange materialxe2x80x9d, which forms all or part of an organic liquid that is substantially immiscible with the aqueous solution. The ionic material combines with the ion exchange material (ion exchanger), forming a compound that is soluble in the organic liquid and substantially insoluble in the aqueous phase. An example of a liquid ion exchanger is a hydroxy oxime ion exchanger, which is useful to extract ionic copper dissolved in acidic or basic aqueous solutions. A hydroxy oxime ion exchanger is sold under the trademark xe2x80x9cLIX 64 Nxe2x80x9d by Henkel Corporation, 1844 West Grant Road, Tucson, Ariz.
With various extractants available today, it is possible to isolate many different soluble materials in a concentrated and pure form from an initially complex dilute solution of the material.
A problem with all liquid-liquid extraction is achieving rapid and efficient separation of the two mixed liquids. Conventional settlers relying on gravity alone must be large to allow adequate residence time to achieve desired separation, and therefore must be large, which requires a lot of space, and expensive construction. Centrifugal separators have been used to accelerate the separation of the two liquids. However, centrifugal equipment available before this invention is expensive, and thus is limited to applications where the recovered material has an inordinately high value, such as pharmaceuticals. In addition, a serious problem with prior art centrifuges is that they permit the formation of an air or vapor phase, which aggravates the problem of entrainment of one liquid in the other, thereby reducing separator efficiency.
Although continuous throughput centrifugal separators have been available for more than 60 years (for example, see U.S. Pat. No. 2,044,996 issued in 1936 to Podbielniak), the industry still needs a cost-effective continuous centrifuge, which rapidly and efficiently separates the mixture into two phases, and with minimum entrainment of one liquid phase in the other.
This invention provides a low cost, effective centrifugal separator for two liquids which are of different densities and are substantially mutually insoluble. In the preferred form of the invention, an efficient mixer section is provided at the inlet end of the centrifugal separator. The mixer section provides such intimate contact between the two liquids that the same material transfer that takes up to two minutes in conventional mixers can take place in less than 10 seconds because of the intimate contact followed by fast separation of the two mixed liquids in the centrifugal separator.
In brief, the apparatus of this invention for separating a mixture of two liquids of different densities and which are substantially insoluble in each other includes a separator tank rotatable about an axis extending through the tank. The tank includes an inlet for the two mixed liquids. The tank also includes an outlet on the axis of rotation for the light liquid, and an outlet spaced from the axis of rotation for the heavy liquid. Thus, as the tank rotates, the heavy liquid is driven outwardly, forcing the light liquid inwardly to the axis of rotation, where the light liquid is removed. Preferably, the tank is secured to and rotates with a drive shaft having a longitudinally extending hollow section collinear with the axis of rotation. One or more radially extending ports through the wall of the shaft permit the light liquid to flow into the hollow section of the shaft and out of the tank. Thus, there is no opportunity for an air column or a vapor phase to form around the axis of rotation. This ensures that all the volume available in the tank for the light liquid is fully utilized, and that no air or vapor is entrained in the liquid.
Preferably, the tank is in the shape of a vertical right cylinder having an annular inlet end wall, an annular outlet end wall, and a cylindrical sidewall. The drive shaft extends through the tank and the two annular end walls along an axis collinear with an axis parallel to the cylindrical sidewall of the tank, and located in the center of the tank. The shaft is secured to the inner periphery of the annular inlet end wall. The inlet for the two mixed liquids is at the outlet end wall of the tank, and the outlets for the two separated liquids are at the outlet end wall of the tank. Preferably, the two mixed liquids enter the tank in the vicinity of the axis of rotation. An annular outlet or deflection baffle in the tank is secured around its inner periphery to the drive shaft adjacent the outlet end wall of the tank between the outlet end wall of the tank and the inlet ports in the shaft for the light liquid. The outside diameter of the annular baffle is slightly less than the inside diameter of the tank. This forces the heavy liquid to travel outwardly, around the outer edge of the annular outlet baffle, and through an annular space between the baffle outer periphery and the inner surface of the tank side wall. A plurality of outlet ports for the heavy liquid extend through the outlet end of the tank wall at various distances from the axis of rotation. The heavy liquid outlet ports can be opened or closed with removable plugs to set the position of the interface or xe2x80x9cneutral zonexe2x80x9d between the two liquids in the tank. Heavy liquid flows from the top of the outlet end of the rotating tank down into an annular space between the tank and a stationary housing around the tank, and is removed for further processing or storage.
In the preferred form of the invention, the drive shaft is supported in a vertical position by a thrust bearing at the upper end of the shaft. The bearing is supported by an upright rectangular frame, which surrounds and also supports the stationary housing disposed around the rotatable tank. Preferably the stationary housing is cylindrical, and coaxially disposed around the rotatable tank. The upper end of the drive shaft is connected to a hydraulic or an electric motor mounted on the frame. The motor rotates the shaft and tank at the desired operating speed. In one form of the invention, the shaft extends from the motor down through a pair of longitudinally spaced rotatable seals at the upper and lower ends, respectively, of a light liquid collector, which is mounted on the top of the housing, and surrounds the drive shaft. A series of light liquid outlet ports extending through the wall of the hollow section of the shaft permit light liquid to flow into the light liquid collector for further processing or to storage.
The lower end of the drive shaft extends coaxially down through a vertical inlet tube secured to the inner periphery of the annular outlet end wall. The shaft is secured to the upper end of the inlet tube above the outlet end wall of the tank. A rotating seal connected to the inlet tube and the housing bottom around the central opening in the bottom of the housing permits rotation of the tank and inlet tube without loss of fluid from the housing, except through a heavy liquid outlet through the outlet end of the housing.
In the presently preferred form of the invention, a mixing unit is secured to the end of the housing adjacent the inlet wall of the tank, and mixes two incoming streams of heavy and light liquids. That mixture is fed through the inlet tube into the inlet end of the rotating tank.
In another form of the invention, the upper rotating seals can be omitted, and the light fluid from the hollow portion of the shaft is collected in a reservoir on the outlet end of the tank housing.
In terms of method, the invention separates two substantially mutually insoluble liquids of different densities from a body of liquid containing a mixture of the two liquids by rotating the body of liquid about an axis to cause the light liquid to concentrate at the axis of rotation, and to cause the heavy liquid to concentrate at the periphery of the rotating body of liquid. The light liquid is removed from the body along a flow path collinear with the axis, and the heavy liquid is removed from the periphery of the rotating body of liquid. Preferably, a mixture of the two liquids is added to the rotating body of liquid as light and heavy liquids are separately removed from the rotating body of liquid.